Anas b. Malik reported that Umm Sulaim narrated it that she asked the Apostle of Allah ﷺ about a woman who sees in a dream what a man sees (sexual dream). The Messenger of Allah (may peace be upon bi m) said: In case a woman sees that, she must take a bath. Umm Sulaim said: I was bashful on account of that and said: Does it happen? Upon this the Apostle of Allah ﷺ said: Yes (it does happen), otherwise how can (a child) resemble her? Man's discharge (i. e. sperm) is thick and white and the discharge of woman is thin and yellow;
so the resemblance comes from the one whose genes prevail or dominate.

حَدَّثَنَا عَبَّاسُ بْنُ الْوَلِيدِ، حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ، حَدَّثَنَا سَعِيدٌ، عَنْ قَتَادَةَ، أَنَّ أَنَسَ بْنَ مَالِكٍ، حَدَّثَهُمْ أَنَّ أُمَّ سُلَيْمٍ حَدَّثَتْ أَنَّهَا، سَأَلَتْ نَبِيَّ اللَّهِ ﷺ عَنِ الْمَرْأَةِ تَرَى فِي مَنَامِهَا مَا يَرَى الرَّجُلُ فَقَالَ رَسُولُ اللَّهِ ﷺ " إِذَا رَأَتْ ذَلِكِ الْمَرْأَةُ فَلْتَغْتَسِلْ " . فَقَالَتْ أُمُّ سُلَيْمٍ وَاسْتَحْيَيْتُ مِنْ ذَلِكَ قَالَتْ وَهَلْ يَكُونُ هَذَا فَقَالَ نَبِيُّ اللَّهِ ﷺ " نَعَمْ فَمِنْ أَيْنَ يَكُونُ الشَّبَهُ إِنَّ مَاءَ الرَّجُلِ غَلِيظٌ أَبْيَضُ وَمَاءَ الْمَرْأَةِ رَقِيقٌ أَصْفَرُ فَمِنْ أَيِّهِمَا عَلاَ أَوْ سَبَقَ يَكُونُ مِنْهُ الشَّبَهُ " .